08-10-2010, 09:34 AM
[attachment=5357]
Introduction
Basics of HVDC
The fundamental process that occurs in an HVDC system is conversion of electric currents from AC to DC (Rectifier) at the transmitting end and conversion from DC to AC (Inverter) at the receiving end.
The rectifier converts the ac power into dc through controlled rectification. The power is then transmitted over the HVDC line to the inverter and converted back to the ac through controlled inversion and fed to the receiving network. Incase of HVDC long distance transmission the HVDC line to the inverter and converted back to the ac through controlled inversion and fed to the receiving network. Incase of HVDC long distance transmission schemes the distance of transmission of power via dc is typically more than 700 km. In case of transmission via a HVDC back-to-back scheme the distance between the rectifier and inverter is practically zero as the rectifier and inverter may be situated in the same building.
Both rectifier and inverter consist of similar converter which can be operated as rectifier or inverter. Thus the term rectifier and inverter are used in the context of particular mode of operation or for a particular direction of flow of power. The direction of flow of power can be changed by reversing the operation of converters i.e. operating rectifier as inverter and inverter as rectifier.
The term controlled rectification and inversion means that the dc output voltage can be precisely controlled electronically. The control of dc side quantities is achieved by varying the firing angle which decides the instant at which the conduction starts in a valve in the converter. By varying the firing angle the dc voltage can be changed from positive to negative range. By controlling both the converters the desired parameters in terms of voltage and current can be achieved and consequently the desired amount of power can be transmitted in the desired direction.
Bipolar operations
• Balanced bipolar mode: In this mode unbalance current flowing through the ground is controlled to a maximum of 10 A.
• Unbalanced bipolar mode: one of the poles In this mode operates in constant current control mode and the other pole transmits the balance overload ed bipole power.
• Monopolar mode with ground return: It utilizing the electrode line and ground electrodes of each station as the return path.
• Monopolar mode with metallic return: It utilizing the line conductors of unused pole as the return path with one ground electrode isolated.
• Transfer between metallic and ground return: Special equipment is included so that the transfer can be made without interruption in the power flow.
• Frequency control: In case generators connected at rectifier end get islanded i.e. operate only with the HVDC bipole or monopole connection, the frequency will be controlled to 50 Hz.
• Sub-synchronous resonance control: In islanded operation, this controller provided at basic converter level to look into the frequency of AC bus and filters out the component related to the torsional vibration of the generator around its critical speed and by modulation of HVDC power, it nullify these oscillation.
Introduction
Basics of HVDC
The fundamental process that occurs in an HVDC system is conversion of electric currents from AC to DC (Rectifier) at the transmitting end and conversion from DC to AC (Inverter) at the receiving end.
The rectifier converts the ac power into dc through controlled rectification. The power is then transmitted over the HVDC line to the inverter and converted back to the ac through controlled inversion and fed to the receiving network. Incase of HVDC long distance transmission the HVDC line to the inverter and converted back to the ac through controlled inversion and fed to the receiving network. Incase of HVDC long distance transmission schemes the distance of transmission of power via dc is typically more than 700 km. In case of transmission via a HVDC back-to-back scheme the distance between the rectifier and inverter is practically zero as the rectifier and inverter may be situated in the same building.
Both rectifier and inverter consist of similar converter which can be operated as rectifier or inverter. Thus the term rectifier and inverter are used in the context of particular mode of operation or for a particular direction of flow of power. The direction of flow of power can be changed by reversing the operation of converters i.e. operating rectifier as inverter and inverter as rectifier.
The term controlled rectification and inversion means that the dc output voltage can be precisely controlled electronically. The control of dc side quantities is achieved by varying the firing angle which decides the instant at which the conduction starts in a valve in the converter. By varying the firing angle the dc voltage can be changed from positive to negative range. By controlling both the converters the desired parameters in terms of voltage and current can be achieved and consequently the desired amount of power can be transmitted in the desired direction.
Bipolar operations
• Balanced bipolar mode: In this mode unbalance current flowing through the ground is controlled to a maximum of 10 A.
• Unbalanced bipolar mode: one of the poles In this mode operates in constant current control mode and the other pole transmits the balance overload ed bipole power.
• Monopolar mode with ground return: It utilizing the electrode line and ground electrodes of each station as the return path.
• Monopolar mode with metallic return: It utilizing the line conductors of unused pole as the return path with one ground electrode isolated.
• Transfer between metallic and ground return: Special equipment is included so that the transfer can be made without interruption in the power flow.
• Frequency control: In case generators connected at rectifier end get islanded i.e. operate only with the HVDC bipole or monopole connection, the frequency will be controlled to 50 Hz.
• Sub-synchronous resonance control: In islanded operation, this controller provided at basic converter level to look into the frequency of AC bus and filters out the component related to the torsional vibration of the generator around its critical speed and by modulation of HVDC power, it nullify these oscillation.